Radio Frequency Identification (RFID) systems represent the next step in automatic identification techniques started by the familiar bar code schemes. Whereas bar code systems require line-of-sight (LOS) contact between a scanner and the bar code being identified, RFID techniques do not require LOS contact. This is a critical distinction because bar code systems often need manual intervention to ensure LOS contact between a bar code label and the bar code scanner. In sharp contrast, RFID systems eliminate the need for manual alignment between an RFID tag and an RFID reader or interrogator, thereby keeping labor costs at a minimum. In addition, bar code labels can become soiled in transit, rendering them unreadable. Because RFID tags are read using RF transmissions instead of optical transmissions, such soiling need not render RFID tags unreadable. Moreover, RFID tags may be written to in write-once or write-many fashions whereas once a bar code label has been printed further modifications are impossible. These advantages of RFID systems have resulted in the rapid growth of this technology despite the higher costs of RFID tags as compared to a printed bar code label.
Generally, in an RFID system, an RFID tag includes a transponder and a tag antenna, which communicates with an RFID transceiver pursuant to the receipt of a signal, such as an interrogation or encoding signal, from the RFID interrogator. As suggested by the name “RFID,” the encoding signal is a transmitted RF signal. The transmitted RF signal causes the RFID transponder to emit via the tag antenna a signal, such as an identification or encoding verification signal, which is received by the RFID interrogator. In passive RFID systems, the RFID tag has no power source of its own and therefore the interrogation signal from the RFID interrogator also provides operating power to the RFID tag.
Currently, a commonly used method for encoding the RFID tags is by way of an inductively coupled antenna comprising a pair of inductors or transmission lines placed in proximity of the RFID transponder to provide operating power and encoding signals to the RFID transponder by way of magnetic coupling. Magnetic coupling, however, is not without shortcomings. Magnetic coupling generally depends on the geometry of the RFID tag, such as the shape of the tag antenna, transponder, etc, so an often complex process for determining an optimal alignment of transceiver with the RFID tag is necessary for effectively directing the magnetic field between the transceiver and the RFID tag such that their magnetic fields would couple. Furthermore, this process has to be redone if the transceiver is be used for encoding an RFID tag of a different geometry, due to a different shape or a different orientation with respect to the pair of inductors when placed in proximity of the RFID transponder.
An attractive alternative to magnetically-coupled RFID encoding schemes are near-field encoders such as capacitively-coupled RFID encoders. For example, U.S. patent application Ser. No. 11/207,222 (the '222 application) filed Aug. 19, 2005, the contents of which are incorporated by reference herein in their entirety, describes a capacitively-coupled RFID encoder. Unlike conventional near-field capacitively-coupled encoders, the encoder described in the '222 application requires no modification to the encoded tag. In contrast, conventional near-field techniques typically require the RFID tag antenna to be modified with capacitive plates. However, the '222 application describes a stripline antenna having enhanced near-field coupling capability such that unmodified RFID tags are readily near-field-encoded. Moreover, the stripline approach described in the '222 application provides inherent impedance matching and optimal non-resonant excitation capabilities.
Despite the advances disclosed in the '222 application, there remain unfulfilled needs in the art. For example, despite the advantages of RFID technology over the bar code arts, users still desire a bar code label on goods identified using RFID tags. Thus, industrial bar code printers have been developed that incorporate near-field RFID encoders as well. As goods pass by such a printer, they receive both an encoded-RFID tag as well as a corresponding bar code label. The un-encoded RFID tags for the near-field encoder are generally stored in a roll akin to a roll of stickers in that the RFID tags typically have an adhesive backing. Conventional RFID tags are sized into approximately 4 inch by 6 inch rectangular shapes. Even if such tags are placed relatively close to one another on their roll, the corresponding centers of RF reception for such conventional tags are thus separated by 4 to 6 inches. Because of this relatively wide separation, the near-field encoder may successfully encode a given tag without encoding neighboring tags.
A problem arises, however, as RFID tag dimensions are continually reduced. For example, RFID tags have been developed that are only ⅜ of an inch in diameter. The relatively wide separation between centers of RF reception on the RFID tag roll discussed previously is thus dramatically reduced. Because the pitch between RFID tags is now much smaller, a near-field encoder has difficulty encoding a given tag on the roll without encoding neighboring tags. Moreover, this problem is exacerbated as the RF sensitivity of modern RFID tags is increased.
Accordingly, there is a need in the art for near-field encoders adapted to encode small pitch RFID tags without stray encoding of neighboring tags.